Coulter CT-100

I have a vintage Coulter CT-100 as a "travel 'scope".
This little jewel is a 4.25" f/4 Newtonian (focal length 432mm)
which disassembles into two main sections: a mirror unit and a
secondary unit. These two main sections have dovetail mounts
which slide onto a single bar (with 1/4" threaded tripod mount)
which comprises the open tube of the telescope. There's a piece
of cloth which velcros around the main sections to baffle stray
light and to keep the optics clean in dirty environments.
It also has a 6x30 finder on a dovetail mount.

When not in use, the two halves nest together and slide into a
case about 6" cubed. The center bar screws onto the shoulder strap
of the case. The cloth tube cover goes on top of the optics, and
the finder and its mount squeeze in on top, so it's all self contained
(except for a tripod). The whole thing weighs about 5 lbs,
considerably less than a Pronto or ETX.

Some people call this the "coffee can scope", because the mirror
and secondary pieces are the same diameter as a coffee can, so close
that the dust covers supplied by Coulter are actual coffee can lids.
It doesn't really look like it's made out of a coffee can, though --
it's light and strong aluminum, blue anodized.

The telescope cost $99, plus about $20 for the optional finder,
back around 1980-81 when it was discontinued.
I bought one of the last units Coulter made: I'd been lusting after
one for some time, and when I heard they were discontinuing it
due to the high demand for their (much more profitable) big
Dobsonians, I drove out to Idlewild to buy one in person.
I've never regretted it.

Customizations

I've made a few minor changes to it, of course (I don't seem
constitutionally capable of keeping anything completely stock).
The worst problem with the CT-100 was that it came without a focuser;
just a set-screw, so you focus either by sliding the eyepiece in and
out of the tube, then locking the set-screw, or by sliding the mirror
or diagonal back and forth on the main truss bar. It's quite difficult
to focus precisely that way.
Fortunately, one of the good design decisions Coulter made was to
put everything together with bolts rather than rivets or welding,
so everything disassembles easily.
So I unbolted the eyepiece holder, bought a used 1-1/4" focuser,
drilled some holes in the right place and screwed it in place.

I've also added a homemade mount for a Daisy unit-power reflex sight.
I'd bought the Daisy sight because I thought they looked like neat
toys and I wanted one. I still think they're neat toys, but in
truth, I like regular finderscopes better (especially the excellent
6x30 which came with the Coulter, which is better than the finder
on my Cave 6", though the dovetail mount isn't as good, only having
one ring instead of two) and I find I never bother to mount the
Daisy sight, or, if I mount it, I never turn it on unless I suddenly
feel the urge to see the neat LED light up.

Finally, the other problem with the CT-100's design is that, having only
one truss bar, the cloth tube cover tends to sag into the optical path.
For a long time I didn't use the cover for that reason, but after the
very dusty PAS Yosemite star party and a damp star party at Fremont
Peak where the mirror dewed badly, I decided that I wanted a cover.
So I got a bunch of bamboo barbecue skewers, which just happen to be
exactly the right length, sewed a lot of loops of thread into the
outside of the cover (a LOT of loops -- five per skewer, for six or
seven skewers; I'm not a great seamstress so this occupied several
hours which weren't nearly as fun as the several hours I spent making
the brightness control for the Daisy sight), and now I have a semi-rigid
tube cover which can't sag into the optical path. When I actually did
the sewing, I didn't have any BBQ skewers handy, so I used brightly
colored plastic pick-up sticks (a birthday gift from a friend --
my close friends obviously have a high degree of respect for my
maturity level. :-) I really liked the look of the pick-up sticks
(they reminded me of those colorful plastic fittings kids put on
bicycle spokes), and they're easier to thread than bamboo because
they don't get splinters, but they weren't quite long enough.
Eventually I plan to look for some longer pick-up sticks.

What can I see through a 4.25" f/4 Newtonian?

When I first got the CT-100, I didn't expect that a telescope so small
could show much. I was wrong. A 4.25" f/4 Newtonian has an
extrodinarily wide field, so you can see views of several objects
in the same field which larger telescopes can't begin to see even
with the most expensive wide-field 2" eyepieces. The Double Cluster
in Perseus is a joy in this telescope. In the summer of 1996,
Jupiter spent several months within a degree or so of the globular
cluster M22, and it was lovely to see Jupiter (with some banding
visible even in the very low-power image), its moons, and M22 all
in the same field. Later that summer during the total lunar eclipse,
with a 40mm eyepiece
I was able to show visitors to Foothill College a view of the eclipsed
moon, a star near the moon's limb, and Saturn, all in the same field.

At the same time, a 4.25" has much more light gathering power than
the small refractors which are today's common travel scopes.
M81 and M82 in the same field are bright and obvious, and a wealth
of smaller galaxies, like NGC 4565, are possible. The view of the
Rosette nebula, with a 25mm Plossl and a UHC filter is the best I've seen
-- bigger scopes gather more light but have focal lengths too long to
show the whole nebula and cluster in the same field.

What doesn't it do well? In a word, planets.
It's hard to get a good image at powers greater than about 60.
Part of the problem may be the relatively large secondary that
such a fast 'scope requires, but I suspect that most of the problem
is collimation difficulty -- the CT-100 does
have collimation screws, but they're of the lock-screw variety
rather than spring loaded, which means it can only be collimated
when pointing more or less straight up, and tightening down the
lock screws can affect the collimation so you have loosen the lock
screw and repeat the process. There's a small amount of slop in
the dovetail mounts where the mirror and secondary units mount to
the truss bar, which presumably also affects collimation, though
I'm still not clear quite by how much. And when I attempted to
star test the 'scope after the last time I collimated it, I noticed
that the out of focus diffraction rings were symmetrical inside of
focus, but noticably comatic outside of focus, which I suspect means
that I didn't get the focuser quite square when I mounted it (which
effect would be magnified outside of focus), a problem which I will
eventually fix.

I'm still relatively inexperienced at both collimation and star testing;
when I become a collimation wizard, I'll figure out how to get this right,
and then perhaps the CT-100 will perform reasonably on planets as well
as deep-sky objects.

Why doesn't someone still sell something like this?

Good question, and one which I frequently ask myself.
The only similar unit that's currently commercially available
is the Edmund Scientific Astroscan, another 4.25" f/4 Newtonian.
However, the Astroscan comes in a bulky plastic package which is
difficult to mount on a tripod or to fit a finder to, has collimation
which is not user adjustable at all, and mounts its secondary via
an optical window of perhaps questionable quality, instead of a
more standard spider. (Actually, the CT-100 doesn't have a standard
four-vane spider either, but rather uses a spider of two thicker vanes,
rather like a scaled-down version the Coulter Dobsonians from that era.)

I think there are good Astroscans; I know several experienced amateur
astronomers who own Astroscans and like them. I have never seen one,
however; I've only had the opportunity to look through a couple of
Astroscans, but every one I've looked through has had pronounced
field curvature or other serious optical problems. I'm not talking
minor subtleties in the shape or number of the diffraction rings,
but rather inability to reach focus on large parts of the field.
I hope it's just coincidence that I happen to have seen bad examples
of the Astroscan, and that most Astroscans are much better than this,
but I can't recommend them based on what I've seen.

Aside from the Astroscan, the only option for a 4.25" RFT is to build
one yourself. Edmund sells Astroscan mirrors for a reasonable price;
I'd like to think that perhaps the problems I've seen with their scopes
is due to collimation or problems in the optical window, and not a
poor quality mirror. I think there are other suppliers of mirrors
this size, but I don't know for sure. Of course, if you're willing
to take the time to grind a mirror yourself, 4.25" and 4.5" blanks
are easy to come by. Building a tube for a 'scope this small should
be easy; or if you want a collapsible backpacking 'scope like the CT-100,
you could even use real coffee cans (I haven't investigated the
availability of parts to make the dovetail and truss bar parts).

Orion also has a rather interesting 4.5" "short-tube reflector" offering.
This is a short focal length 4.5" mirror (I'm not sure exactly what the
focal ratio is) with a built-in barlow at the eyepiece to double the
focal length. No one seems to know for sure how the optics are on
this 'scope, and it's not clear whether the mirror might be usable
without the built-in barlow - it may be that the mirror is spherical
and the barlow section includes a correction for spherical aberration,
in which case the short-tube would not be usable as an RFT without the
barlow even if its optics are good with the barlow.

The rest of the "travel scope" options are either refractors, high-end
ones like the Tele-Vue Ranger or Pronto or low-end ones like the
Celestron 80mm or the Orion 80mm short-tube refractor; or catadioptrics
like the Meade ETX, Celestron C90 or C-5, or the
Pro-Optic/Orion 90mm f/5.6 Maksutov.